Memory devices provide data storage for electronic systems. One type of non-volatile memory is known as flash memory. A flash memory is a type of electrically-erasable programmable read-only memory (EEPROM) that may be erased and reprogrammed in blocks. Many modern personal computers use a BIOS stored in flash memory, sometimes called a flash BIOS. Wireless electronic devices may use flash memory since it enables a manufacturer to support new communication protocols as they become standardized and to remotely upgrade the device. Other uses for flash memory include USB flash drives, memory cards, digital audio players, digital cameras, GPS/navigation devices, etc.
Often, a flash memory includes a memory array that contains memory cells arranged in row and column fashion. A memory cell may include a floating gate field effect transistor capable of holding a charge, with multiple cells usually grouped into blocks. Cells within a block may be electrically programmed by charging the floating gate. The charge may be removed from the floating gate by a block erase program.
In a single level cell, the cell value may be determined by the presence or absence of the charge on the floating gate. In a multilevel cell, the cell value may be determined by the amount of charge on the floating gate in addition to the absence of charge. That is, the threshold voltage (Vt) of a cell decreases with increasing charge. Accordingly, when sensing the presence of charge, a higher range of threshold voltages may be indicative of one charge level while a lower range of threshold voltages may be indicative of a higher charge level. The different charge levels represent different stored values.
NAND is a basic architecture of flash memory, although a NOR architecture may instead be used. A NAND cell unit includes a select gate coupled in series to a serial combination of memory cells (with the serial combination often referred to as a NAND string). With the cells serially combined in a string, understandably, scaling NAND memory cells may bring them in very close proximity. Interference between cells in the form of capacitive coupling between floating gates may produce inaccurate sensing of threshold voltages and, thus, charge levels. Such phenomenon is reported in Lee, et al., “Effects of Floating-Gate Interference on NAND Flash Memory Cell Operation,” IEEE Electron Dev. Let., Vol. 23, No. 5, May 2002, pgs. 264-266.
Clearly then, a need exists in the art for improved NAND memory cells and methods of forming NAND memory cells.